1. Field of the Invention
This invention relates to multichannel optical wavelength multiplexers/demultiplexers. Accordingly, it is a general object of this invention to provide new and improved multiplexers/demultiplexers of such character.
2. General Background
An optical birefringent filter of the type first described by Solc in 1953 and reviewed in his article "Birefringent Chain Filters", Journal of the Optical Society of America, Vol. 55, No. 6, pp. 621-625 (1965), is particularly adaptable for use in wavelength division multiplexers and demultiplexers because it consists of a substantially lossless network of birefringent crystal elements between two polarizing devices. Its transmission characteristics are periodic functions of optical frequency that can be shaped as desired by choosing an appropriate number of equal length elements and their rotational orientations according to a synthesis procedure outlined by S. E. Harris, E. O. Ammann and I. C. Chang, "Optical Network Synthesis Using Birefringent Crystals. I. Synthesis of Lossless Networks of Equal-Length Crystals", Journal of the Optical Society of America, Vol. 54, No. 10, pp. 1267-1279 (1964). A single birefringent element between two polarizing beam splitters to make a polarization insensitive wavelength demultiplexer that is useful in fiber optic systems has been described by P. Melman, W. J. Carlsen and B. Foley, "Tunable birefringent wavelength-division multiplexer/demultiplexer", in Electronic Letters, Vol. 21, No. 15, pp. 634-635 (1985). There, they describe their splitting of an input beam into two orthogonal plane polarized components that pass in parallel through the birefringent element and are then combined in a second polarizing beam splitter to yield two output beams consisting of the input light separated according to wavelength.
Carlsen and Melman describe an n channel demultiplexer configured as a tree structure of n-1 two channel demultiplexers each with a single birefringent element in their U.S. Pat. No. 4,566,761 referred to hereinabove. As shown therein, each stage would have a sinusoidal transfer function, and successive stages would have halved periods. An input beam with n wavelength channel components would be demultiplexed into two, four, and finally n output beams, each containing only one wavelength channel. Successive stages would consist of two polarizing beam splitters between which would be located a birefringent network with an appropriate number of elements to achieve the desired transfer function.
Related art to the present invention includes a configuration disclosed by B. M. Foley in her application Ser. No. 018232 referred to hereinabove. It includes successive stages of birefringent filters, each including a sequential network of birefringent crystal elements between polarizing beam splitters. In a demultiplexer mode of operation, they separate an input beam into four output beams. Each output beam contains light wavelengths in only one of the optical wavelength channels determined by the transmission functions of each filter. The overall device is reversible, and, in the reverse direction multiplexer mode, four separate beams enter the device, merge, and exit as a single beam, provided that each of the separate beams consists only of light within the optical wavelength channels determined by the same transmission functions.
In terms of its operation as a demultiplexer, the input beam enters a beam splitter prism where only a first vertically polarized component is reflected by a dielectric multilayer coating, and is thereby separated from a second horizontally polarized component which continues into an initial sequential network of eleven birefringent crystal elements. The first component beam is again reflected internally in the prism at a facet which is parallel to the plane of the coating. It, then, also traverses the initial sequential network parallel to the second component. After undergoing wavelength dependent polarization transformations, both enter a second beam splitter where the first component is again internally reflected and recombined with the second component at a dielectric multilayer coating associated with the second beam splitter. Vertically and horizontally polarized components of each of the first and second components are reflected or transmitted respectively by this latter polarizing coating. Wavelength components of the input beam that fall within separate channels A and B (FIG. 1a) undergo essentially no net change in polarization state in traversing the sequential network and appear in one beam, while wavelength components that fall within channels C and D (FIG. 1a) are essentially converted to the opposite polarization state and appear in a different beam.
The second filter stage functions in essentially the same manner except that it is rotated in orientation by 90 degrees such that its parallel input beams are processed independently. Each is separated by another input polarizing beam splitter into horizontally and vertically polarized components which traverse a sequential network of five birefringent crystal elements and recombine in still another output polarizing beam splitter. Wavelength components of input beams that fall within channels A and C of FIG. 1b undergo essentially no net change in polarization state in traversing the latter filter stage and appear in two of the output beams, while wavelength components that fall within channels B and D are essentially converted to the opposite polarization state and appear in two other output beams. Together, the two stages separate wavelength components of the input beam within the four channels into four separate output beams.
An alternative geometry for a four channel birefringent multiplexer/demultiplexer includes two stages that function in the same way as described above, but the second stage is not rotated relative to the first. Instead, it is made broader to accommodate four parallel beams in the same plane as the two beams in the first stage filter.